When steel is produced using an electric arc furnace, about 20 kg of dust is formed per (metric) tonne of steel produced.
This dust is the result of at least two reactions: in a first reaction, the more volatile metals pass into the vapor phase at the operating temperature of the furnace. When sucked into the air, they become oxidized, and they cool down, thus ending up either as free oxides, or else in the form of composite structures including iron oxides.
In a second reaction, fine particles of iron are dispersed in the vapor phase over the melt of boiling steel and are entrained by air suction. In the flow of air they cool down and under the action of oxygen in the air they are transformed into higher oxides. The interaction between such oxides and heavy metals can lead to the formation of compounds of the spinel type MFe.sub.2 O.sub.4 (for M=Zn, Ni, Mn, or Cd). If M=Fe, then the phase is made of magnetite.
Dust from electrical steelworks thus contains varying quantities of majority elements such as iron, zinc, calcium, and silicon, in the form of single or multiple oxides, together with minority elements such as copper, manganese, chromium, cadmium, lead, and chlorides. The dust is considered as being toxic because heavy metals are salted out by leaching, so the dust is classified as class 1 waste. In this context, European regulations specify that from the year 2002, only waste that is known and confirmed as being "ultimate" will be accepted in class 1 dumps.
Two types of recycling method have been considered for processing dust from electrical steelworks.
A first type consists in performing reduction reactions at various temperatures depending on the particular method, to volatilize the heavy metals and reinject the ferrous fraction into the steel production furnaces.
Thus, document EP-A-336 923 (U.S. Pat. No. 4,940,487) proposes processing foundry dust in an iron smelting furnace after the dust has been transformed into pellets; document EP-A-441 052 (U.S. Pat. No. 5,188,658) proposes heat treatment in the range 1200.degree. C. to 1700.degree. C. while adding reducing agents. Document WO-A-91 12 210 (U.S. Pat. No. 5,364,441) describes a method of processing foundry dust in an iron smelting furnace, with the heavy metals being recovered by condensation from the hot gases; document EP-A-453 151 describes a method of processing dust in the form of pellets by using an agent that selectively reduces iron oxide; document FR-A-2 666 592 (U.S. Pat. No. 5,180,227) describes apparatus for extracting volatile metals by oxidation; document EP-A-3 193 describes an electric arc furnace specially designed for reducing dust that contains oxides; document EP-A-6 538 717 relates to a pyrometallurgical method associated with a step of distilling heavy metals in the liquid state; and document EP-A-551 992 (U.S. Pat. No. 5,279,643) describes a method of recovering recyclable metals from foundry dust by induction and volatization of the recyclable metals.
A second type of method consists in directing the dust to a use where it is rendered inert by various means for use as a building or filler material.
Thus, document EP-A-402 746 describes a recycling method in which foundry dust is integrated in clay to make a material for packing mines. Document WO-A-91 122 10 (U.S. Pat. No. 5,364,441) proposes using foundry dust for processing sewage with the addition of a flocculating agent.
Finally, document FR-A-2 689 881 (U.S. Pat. No. 5,278,111) describes a method of manufacturing bricks having properties that are improved by adding electrical steelworks dust that has previously been calcined at the volatilization temperature of the heavy metals contained in said duct.